Field of the Invention
This application relates generally to optical communications based on optical wavelength-division multiplexing (WDM), and in particular to subchannel routing, switching, and protection, along with related techniques that facilitate network upgrades and reuse of legacy equipment.
Description of Related Art
1. Overview
Optical WDM communication systems transmit multiple optical channels at different WDM carrier wavelengths through a single fiber. The infrastructures of many deployed optical fiber networks today are based on 10 Gb/s per channel. As the demand for higher transmission speeds increases, there is a need for optical networks at 40 Gb/s, 100 Gb/s or higher speeds per channel.
Moreover, there is a need to leverage this higher bandwidth to realize greater flexibility in routing client signals among network nodes. For example, increasing the bandwidth of a fiber channel from 10 Gb/s to 40 Gb/s might enable 4×10 Gb/s client circuits to occupy a channel between two network nodes previously dedicated to a single 10 Gb/s client circuit. Yet, unless an entire channel is free to enable all four client circuits to continue propagating together to a subsequent node on the network, the desired routing of these four client circuits may not be achievable without some mechanism for dynamically rerouting individual client circuits, independent of one another, across different fiber channels.
As will become apparent below, there is a need not only for increased bandwidth, but for sufficient flexibility to divide and/or combine individual client circuits to achieve desired routing, switching, concatenation and protection capabilities. Such flexibility is needed to fully realize the benefit of increasing the number of available optical circuits in a single fiber.
2. Single-Wavelength Optical Networks
Optical fiber has been used as a communication means since about 1977. Over time, deployed baud rates on a single laser have increased from 45 MB/s to over 40 Gb/s. Various protocols have been transmitted across optical fiber, including SONET [GR-253] and Gigabit Ethernet [IEEE Standard 802.3ae].
FIG. 1A shows a deployed network 100 that uses OC-48 SONET add-drop multiplexers 120 at each node, interconnected by a first fiber optic cable 125 for signals traveling in a clockwise direction, and a second fiber optic cable 135 for signals travelling in a counterclockwise direction. At each node (or network add/drop site) lower-rate client traffic 110 can be added or dropped, or passed through that node. SONET mappers are used to map the traffic to the STS-1 virtual containers [described in Telcordia Standard GR-253], and SONET multiplexers are used to direct the traffic to the add, drop, or passthrough ports. A pair of multiplexers can be used on two separate line cards as shown to provide support for a Unidirectional Path Switched Ring (UPSR), or a 2-fiber or 4-fiber Bidirectional Line-Switched Ring (BLSR). [GR-1230 Telcordia Standard describes the SONET BLSR]. The traffic from a SONET ADM can also be combined with other traffic using wave-division multiplexing (WDM) to increase the network capacity.
FIG. 1B shows a deployed network 150 that uses Gigabit Ethernet switches 170 at each node, interconnected by a first fiber optic cable 175 for signals traveling in a clockwise direction, and a second fiber optic cable 185 for signals travelling in a counterclockwise direction. At each node incoming Gigabit Ethernet traffic 160 is mapped to VLANs that are transmitted on the 10 GE line side. At each node traffic in each VLAN is selected to be added or dropped, or passed through that node. When GE networks are deployed in a ring, the standard protocols of STP (Spanning-Tree Protocol) and RPR (Resilient Packet Ring) can be used to provide protection. The traffic from a 10 GE switch can also be combined with other traffic using WDM.
3. WDM networks with Muxponders and Transponders
Later generations of optical fiber communication systems use optical amplifiers to increase span and repeater distances and wavelength-division multiplexing to increase the link capacity or aggregate bandwidth. WDM networks transmit client traffic from multiple sources over an optical fiber network. The traffic is multiplexed on the fiber by transmitting each signal with a laser set at a different channel on the International Telecommunication Union (ITU) channel plan defined in Standard G.692. Optical filters designed to function according to the ITU channel plan are used to demultiplex the signals and thereby direct each signal to its designated receiver. These standard ITU channels are hereinafter referred to simply as “channels.”
Optical signals are transmitted using transponders or muxponders, and are demultiplexed with fixed optical add-drop multiplexers (FOADMs), reconfigurable optical add-drop multiplexers (ROADMs), and/or wavelength selective switches (WSS).
FIG. 2 shows a currently deployed WDM transponder 200. Client traffic 210 is connected via a short-reach fiber interface to client transceivers 215. These are typically pluggable devices such as an XFP [MSA standard http://www.xfpmsa.org/cgi-bin/msa.cgi]. After the optical signal is converted to an equivalent electrical signal (utilizing clock recovery circuitry 218), it can be processed digitally to optionally (1) extract performance monitoring information 220, (2) add channel overhead for remote network management 225, and (3) encode the data for forward error correction 227. The signal is then used to modulate light from a fixed or tunable laser on the WDM grid. The output 230 from the transmitter 229 is then launched onto the transmission fiber. The transmitted light signal can be combined with light signals from other WDM transponders on a single fiber with an optical multiplexer.
At the receive side of the link, an optical demultiplexer is used to separate the WDM signals 235 (on the incoming fiber), which are then converted back into equivalent electrical signals by the receive circuitry 237 in the transponder. Note that this transponder requires external means to select the particular wavelength that is being dropped, though this filter function can be integrated onto the transponder line card [see, eg, U.S. Pat. No. 6,525,857]. The electrical signal from the line receiver (utilizing clock recovery circuitry 239) can be processed digitally to optionally (1) extract performance monitoring information 241, (2) drop the channel overhead for remote network management 225, and (3) correct errors according to the Forward Error Correction (FEC) algorithm 243. The signal 240 is then returned to the client equipment via the client-side transceivers 215. As alluded to above, transponders may utilize clock recovery circuitry 239 to support different data rates and protocols.
Typically, the line side optics are designed to operate at 2.7 Gb/s, 10.7-11 Gb/s, or 43 Gb/s with the cost of the components increasing with bit rate. The line receiver 237 is either a PIN photodiode or avalanche photodiode. In either case the receiver is not wavelength specific, so that an optical demultiplexer, or ITU channel filter, must be placed in front of the receiver to filter out the designated channel.
It should also be noted that control plane circuitry and software 250 is employed to facilitate various transmit and receive functions of DWDM transponder 200, such as remote network management 225 (e.g., via the addition or removal of channel overhead) and the extraction of performance monitoring information 245. In addition, control plane 250 is employed for configuration of transmission protocols 255 (in concert with clock recovery circuitry 218) and laser wavelengths 265 (to tune channels via transmitter 229). Finally, it can detect and handle faults involving the reception of both client-side (267a) and line-side (267b) signals.
FIG. 3 shows a currently deployed WDM muxponder 300. This module maps lower-rate traffic 310 using a SONET multiplexer [GR-253], OTN (Optical Transport Network) multiplexer [based on ITU standard G.709], Ethernet switch, or proprietary digital mapping and multiplexing 320. The multiplexing may be done with a commercially available or custom-designed ASIC, or a custom-designed FPGA. The muxponder 300 has line-side WDM optics similar to the transponder 200 with a laser (in transmitter 329) set to a designated channel on the ITU grid and a receiver 337 that can detect any signal within the ITU channel plan.
Although the transponder 200 and muxponder 300 can be designed to transmit signals from different sources and with different bit rates, the hardware limitations and costs typically limit the implementation to a specific set of protocols. For example, a 10 Gb/s transponder may transmit OC-192 or S™-64 signals at 9.95 Gb/s, 10 GbE signals at 10.3125 Gb/s, FC-10 signals at 10.5 Gb/s, and OTU signals at 10.7 Gb/s. But it may not transmit data at significantly different data rates such as 2.5 Gb/s or 1.25 Gb/s. This may be a limit of the clock-recovery circuits used, SERDES (serializer-deserializer) circuits, or the ASIC or FPGA used to perform the performance monitoring and FEC functions. Similarly, a muxponder typically supports a subset of data rates and protocols that are determined by the capabilities of the digital and analog electronic circuits. The maximum data rate supported by the transponder and muxponder is typically limited by the analog circuits on the line side, such as the optical modulator (or bandwidth of the laser if direct modulation is being used), the bandwidth of the optical receiver, and the bandwidth of the transimpedance amplifier used at the receiver.
WDM network installations have been a compromise between price and functionality. The cost of the high-speed optics increases with the line bit rate so that vendors typically partition their products into different data rates such as 2.5 Gb/s, 10 Gb/s, 40 Gb/s, and 100 Gb/s. The price of WDM ports dictates that networks be deployed with as much bandwidth per port as possible. However, this has been offset by transponder prices increasing with bandwidth (e.g. 40G) so that most WDM lambdas have bandwidth assignments that are “right sized.”
4. WDM Channel Plans
WDM network channel plans usually use a subset of the wavelengths on the International Telecommunication Union, Telecommunication Sector (ITU-T) grid. Reference Standard G.692, which specifies a frequency grid anchored at 192.1 THz with interchannel spacings at integer multiples of 50 GHz and 100 GHz, is specified as the basis for selecting channel central frequencies. For purposes of illustration, the ITU channels will be referenced as 1-210 for 192.1 THz, 1-215 for 192.15 THz, etc.
The number of channels on the ITU grid is limited in most applications to the gain range available from erbium-doped fiber amplifiers (EDFAs). Gain-flattened EDFAs are now commercially available for either the C band (˜191.8 THz to 196.3 GHz) and L band (˜186.9 to 191.4 GHz). Currently a fully-loaded WDM network can transmit approximately 160 channels—80 channels in the C band spaced at 50 GHz and 80 channels in the L band spaced at 50 GHz.
5. Point-to-Point WDM Links
FIG. 4 shows a simplified diagram of a point-to-point WDM network 400 interconnecting two nodes—East Node 410 and West Node 420. Typically two fibers are used—one to transmit from east to west 402 and one to transmit from west to east 404—but a single fiber can also be used. Signals from different WDM lasers are combined via WDM Combiner 415 that can be wavelength-dependent (using ITU channel filters) or wavelength-independent (using a 1:N optical splitter).
A 1:N optical splitter can be based on fused fiber couplers. It has N input ports and one output port so that optical signals connected to the input ports are combined in the output fiber with a nominal power loss of 10*log 10(N) dB for each signal. At the receive side the signals are demultiplexed via WDM Demultiplexer 417 using optical filters such as thin-film filters.
6. WDM Ring Networks
WDM signals can be transmitted over other topologies, such as a ring 500 shown in FIG. 5. In this example, and several of the examples to follow, a single wavelength at each node has been used to simplify the example. This does not preclude the generalized case of an arbitrary number of wavelengths at each node. The ring topology offers the advantage of having two diverse routes between any nodes on a ring so that any failure on one side of the ring can be protected with redundant traffic on the other side of the ring. WDM equipment networks also support channel passthrough at a node—if an optical filter is not used to drop a particular signal at that node, then the signal continues around the ring to the next node.
Optical filters may be configured to selectively drop channels at a node. In this case the dropped wavelengths can be reused for other signals on the next ring segment. This filter configuration is shown in FIG. 5 where all signals on ring 500 are directed to a filter (or plurality of filters 515, 525, 535 and 545) to select the dropped channels. Currently deployed WDM networks route signals using fixed or reconfigurable optical add-drop multiplexers. In this example, Node 1 510 is configured to drop I-200, Node 2 520 is configured to drop I-210, Node 3 530 is configured to drop I-220, and Node 4 540 is configured to drop I-230.
WDM signals may also be transmitted on a ring in a broadcast and select configuration [see, eg, U.S. Pat. No. 7,120,359]. In this configuration shown in FIG. 6, a portion of the powers of all signals is tapped off at a node and directed to a filter (or plurality of filters 615, 625, 635 and 645), to select the dropped channels. This implementation uses a wideband tap coupler (616, 626, 636 and 646) that directs a fixed fraction of all signals to a drop port. In this case all signals continue around the ring 600 so that the dropped wavelength cannot be reused since it would have interference from the passthrough portion of the signal entering the node. Furthermore, the return path of the signal on the protected side of the ring requires a separate wavelength to avoid interference.
Although the broadcast and select configuration does not support channel re-use, it has the advantage that it supports drop and continue traffic, i.e. traffic that is dropped at more than one node. It also has the advantage that once the tap coupler is in place, adding a filter to drop more channels does not interrupt the passthrough channels. To date, broadcast and select architectures have been limited by the number of channels supported by the optical demultiplexers.
Note that in either configuration of FIG. 5 or FIG. 6, the drop filter may not provide enough isolation on the passthrough path. In that case, the drop filters can be cascaded to improve the isolation. Further note that the diagrams only show one channel dropped at each node. Typical installations cascade filters at each node, or use a parallel filter, to drop more than one channel at each node.
Another advantage of the broadcast and select architecture is that it does not reduce the bandwidth available on the line fiber. All optical filters have a useable passband less than ideal because of the finite slope of the filter passband. The useable bandwidth of cascaded filters decreases as more filters are inserted in the signal path. The graph 700 in FIG. 7 shows the bandwidth narrowing effect for the case where commercially available WSS filters are cascaded in a network. Even though this WSS is designed for 100 GHz ITU channels, it has a useable passband of 68 GHz. Architectures that minimize the filter insertion in the optical path therefore have a higher available cumulative bandwidth.
WDM equipment is typically deployed in an equipment shelf that separates the add/drop functionality from the transponders or muxponders. This offers the service provider the benefits of paying as they grow, especially since the major expense can be the transponders and muxponders. This partitioning of WDM equipment 800 is shown in FIG. 8. A practical implementation would use optical fiber patchcords (802a and 802b, and 804a and 804b) to connect the discrete modules (add/drop modules 810 and 820, and transponders/muxponders 830 and 840), but optical backplanes have also been deployed.
Note that in FIG. 8 filters 815 and 825 are installed with larger channel counts than are being used. Over time channels can be added to the unused filter ports without interrupting the live services. Because of human error in manually adding the fiber patchcords (802a and 802b, and 804a and 804b) between the modules (add/drop modules 810 and 820, and transponders/muxponders 830 and 840), this partitioning can lead to misconnections of the fiber patchcords. Instead of properly connecting module 810 to transponder 830 (via fiber patchcords 802a and 802b), as shown in FIG. 8, these fiber patchcords could mistakenly be misconnected as shown in WDM equipment 900 in FIG. 9—e.g., connecting corresponding module 910 to transponder 940 (instead of transponder 930) via corresponding fiber patchcords 902a and 902b. Similarly, module 920 is mistakenly connected to transponder 930 (instead of transponder 940) via fiber patchcords 904a and 904b. These mistaken connections may be difficult to detect, especially if there are two redundant paths between the terminals.
Tracing optical connections can be difficult in this scenario because the multiplexers, amplifiers, and other equipment may not have means to independently detect each incident channel. U.S. Pat. No. 5,513,029, however, discloses a method whereby an optical signal is modulated with a low frequency dither signal to provide a modulated optical signal having a known modulation depth. A portion of the optical signal is tapped, and both a total power and a dither amplitude of the tapped portion of the optical signal can be measured within the network to provide power levels of the signals. But this method requires dedicated hardware at all monitoring points and it cannot detect third-party signals (i.e., “alien” signals that are generated by other equipment vendors, but which may be inserted into a WDM network as long as they are on the same ITU channel plan and do not interfere with other signals).
Another method that can be used to monitor signals in the network is to deploy optical spectrum analyzers at various points in a network. These can be accessed through the network management software. However, getting a full view of the network may require many of these and thus add considerable cost [see, eg, U.S. Pat. No. 7,002,697]. So there remains a need to improve the end-to-end visibility of signals in a multichannel optical network.
7. Link Budget Rules
To maintain signal integrity and guarantee a high quality of service, optical networks transmitting digital signals must maintain a minimum bit error rate (BER). Well-known factors affecting the WDM signal BER are received power levels, optical signal-noise ratio (OSNR), chromatic dispersion (CD), polarization-mode dispersion (PMD), and nonlinear fiber penalties such as cross-phase modulation and four-wave mixing. Network design rules determine the link budget (maximum distance and loss per fiber span) based on these penalties.
Guaranteeing the performance and determining the link budget for an installed network can be costly because determining the factors listed above may require expensive test equipment. Furthermore, the tests may have to be run while the network is out of service so that changes over time after a network is installed cannot be detected. There is therefore a need to measure the optical parameters for an installed network, with minimal service disruption, and minimal extra cost.
8. Optical Protection
Optical networks often require protection against equipment failures or fiber cuts. One good way of protecting traffic is to provide two or more redundant paths between the end points with a protection algorithm that selects traffic from one of the redundant paths. Standard protection algorithms are the SONET Unidirectional Path Switched Ring (UPSR) and Bidirectional Line-Switched Ring (BLSR). The UPSR provides dedicated protection (each working circuit has a protection circuit of equal bandwidth) and the BLSR provides shared protection (the protection bandwidth equals the total working bandwidth in a single fiber).
Dedicated and shared protection both require means to (1) bridge traffic at the transmit end onto the redundant paths, and (2) select traffic at the receiver from one of the redundant paths.
Shared protection also requires a means to manage the passthrough traffic at the intermediate nodes. Examples of shared protection schemes can be found in U.S. Pat. Nos. 7,046,619 and 7,499,647, and U.S. Pat. App. No. 2007/0025729.
Various forms of optical protection have been proposed and implemented, with the most common being a dedicated 1+1 protection with a 1×2 optical switch in front of the receiver. Although shared optical protection offers significant bandwidth savings, its practicality is limited by the requirement of maintaining link budget rules for all possible protection paths.
9. Larger WDM Networks and WSS
FIG. 10 shows a typical network layout 1000 of a service provider. The ring topology is commonly used in WDM networks because it provides the lowest-cost means of offering protected services. A ring network requires that all nodes have at least two connections to separate neighboring nodes. Ring networks may have spurs hanging off them to serve a small number of isolated nodes that have only a single fiber span connected to another node. Ring networks may be interconnected as shown in FIG. 10 with traffic passing between the rings at one or more intersecting nodes (nodes A and B). Many deployed networks with similar layouts need to pass traffic from a spur to a node on the same ring (node C to D), from a spur to a node on a different ring (node C to E), and between nodes on different rings (node D to E). A 1×N Wavelength-Selective Switch (WSS) can be used to direct traffic between N nodes with direct optical connections [see, eg, U.S. Pat. No. 7,492,986].
WSS-based filters are currently much more expensive than fixed filters. Ring interconnections can also be done with fixed optical filters, but those connections cannot be reconfigured remotely, so that network upgrades require technicians to go to the interconnecting sites and manually reconfigure the connections on the fixed filters.
Furthermore, many deployed optical networks have difficulty managing WDM traffic through on these paths so that the traffic may be segmented by electro-optical conversions at the intersecting nodes (A, B, F). These electro-optical conversions add cost and complexity to the network while reducing reliability. However, all-optical routing between rings and from spurs to rings requires that the network be engineered so that the link budget rules are met by the intra-ring signals, and that individual powers be managed at the intersecting nodes.
10. Subchannel Multiplexing
Various forms of subchannel modulation have been proposed as a means to reduce the dispersion penalties associated with high bit rate transmission in optical fibers (see, eg, WO 2009/105281) and increase spectral efficiency (see, eg, U.S. Pat. No. 6,525,857). These “subchannels” (eg, subchannels of ITU channels) are typically generated by microwave modulators or comb generators with a single laser. Examples of optical comb generators are described in U.S. patent application Ser. No. 12/175,439, entitled “Optical Wavelength-Division-Multiplexed (WDM) Comb Generator Using a Single Laser” and filed on Jul. 17, 2008, which is incorporated by reference herein. These subchannels are closely spaced relative to the source laser and are not independently tunable across a wide wavelength range, i.e. they are tuned in parallel as the source laser is tuned. Although an embodiment of one of the previously referenced patent applications (WO 2009/105281) proposes the use of more than one laser to generate the subchannels, such lasers are constrained to operate in parallel within a single ITU G.692 window.
Lower-rate subcarriers support a simplified upgrade of an installed DWDM network. For example, a legacy 2.5 Gb/s network may have transmitters with a reach of 600 km. When that network is upgraded to 10 Gb/s, dispersion compensators may have to be installed, since the reach of the 10 Gb/s transmitter may be only 80 km. Installing dispersion compensation and amplifiers to compensate for their loss can be very disruptive since operators may have to break the traffic multiple times and at multiple sites. If four subcarriers are used instead, with each subcarrier transmitting at 2.5 Gb/s to get 10 Gb/s composite bandwidth, they can have comparable dispersion-limited reach to the installed 2.5 Gb/s channels. The use of subcarriers therefore provides system operators with a means of upgrading an installed WDM network to increase the network capacity without having to change the dispersion map.
There is thus a need for an improved implementation of subchannels (eg, using independently tunable lasers to generate independent subcarrier frequencies) that will not only increase bandwidth and spectral efficiency by enabling multiple client circuits to be assigned to respective subchannels of a single ITU channel, but will also allow those client circuits to be divided and/or combined with one another and assigned independently to subchannels within and across ITU channels. Such flexibility is needed, as noted above, to achieve desired routing, switching, concatenation and protection capabilities, and thus fully realize the benefit of increasing the number of available optical circuits in a single fiber.
11. Network Upgrades
Even with the ability to upgrade the capacity without installing additional dispersion compensators, adding or removing channels from a DWDM network can be disruptive to the live traffic because the channels can propagate through shared components such as amplifiers and attenuators that act upon the total power. For example, if an attenuator output is being controlled to a certain output power, doubling the channel count will cause the power per channel to be cut in half. This drop in power could cause bit errors. System operators have a need therefore for control (eg, via software) over channel changes in a WDM network in a manner that is minimally disruptive to the live channels.
12. Management Cards
WDM network equipment (e.g., equipment 1100 shown in FIG. 11) is typically installed in a shelf 1110 with one or two management cards 1120 (MGT) and various line cards 1125. The equipment 1100 is typically managed with a client-server element management system (EMS) consisting of one or more clients, such as client 1130, and EMS Server 1140. The EMS connects through a private or public IP network (via Router 1150) to the management cards 1120.
FIG. 12 illustrates how two management cards 1220a and 1220b in equipment shelf 1200 can be deployed in an active/standby configuration to improve network robustness. The standby MGT 1220b takes over the management function if there is any hardware or software failure on the active MGT 1220a. This configuration typically uses two ethernet planes (1235 and 1245) on the backplane so that any line card can communicate with either management card. A handshaking protocol between the management cards is used to determine which is the active MGT at any given time. On each line card there is a switch to select which ethernet bus is used for communications.
This configuration requires control of the software versions running on the MGT microprocessors. They run the same version to ensure compatibility in the event of a switchover from active to standby. The configuration and status databases on the operative MGT are constantly backed up on the backup MGT so that when a failure occurs the backup MGT can take over the management as quickly as possible, and without any service interruptions.
13. OSC Options and Routing Protocols
WDM equipment typically requires that the EMS have a management connection to all remote nodes for functions such as provisioning equipment, reporting faults, downloading software upgrades, and retrieving and reporting performance metrics. The MGT also employs a management connection to remote nodes for end-to-end provisioning, controlling protection switching, and reporting remote performance and faults. For these functions, current WDM equipment deploys an optical service channel (OSC) that is outside of the ITU-T G.692 spectral window, i.e. at 1510 nm or 1620 nm.
Control messages and status can be transmitted from the MGT card to the OSC card over the backplane, and then transmitted optically by the OSC to the remote node where it is routed to the remote MGT card over the remote backplane.
Adding the filters to add and drop the OSC channel add loss and cost to the network. The OSC can be eliminated if channel overhead is inserted into the signals, but the typical channel overhead bandwidth (500 kb/s) is much lower than the typical OSC channel bandwidth (100 Mb/s). There is therefore a need for improved in-band communications channels that provide the necessary bandwidth without adding cost.
14. Optical Switches Interconnected with WDM Links
Switching matrices are used in a telecommunications network to direct traffic from multiple inputs to multiple outputs. An electrical crossbar switch has a matrix of switches between the inputs and the outputs. If the switch has M inputs and N outputs, then a crossbar has a matrix with M×N cross-points or places where the “bars” cross. A given crossbar is a single layer, non-blocking switch. Collections of crossbars can be used to implement multiple layer switches. A Clos network is a kind of multistage switching network, first formalized by Charles Clos in 1953 [see, eg, Charles Clos (March 1953), “A study of non-blocking switching networks,” ‘Bell System Technical Journal’ 32 (5): 406-424]. The Clos network provides a practical multi-stage switching system that is not limited by the size of the largest feasible single crossbar switch. The key advantage of Clos networks is that the number of crosspoints (which make up each crossbar switch) required can be much fewer than if the entire switching system were implemented with one large crossbar switch. Although VLSI technology has enabled very large switching matrices in electronics [see, eg, U.S. Pat. No. 6,714,537], the switch size is still limited at very high bandwidths.
WDM links can be used to interconnect large electro-optic switches, as illustrated in FIG. 13. Optical crossconnect switches based on MEMS [see, eg, U.S. Pat. No. 6,574,386] have also provided a means of switching at the optical layer, but these switches may need wavelength demultiplexers to switch individual wavelengths. Large crossconnect switches 1310 provide the connectivity required to support large traffic demands and WDM links 1320 provide the bandwidth between the switches.
Note that this architecture shown in ring 1300 can be costly because O-E-O conversions may be required at each switch and bandwidth is being used to send traffic to and from the centralized switches. Also, the cost of such switches increases with the number of ports and bandwidth per port so that a network based on switches that support traffic bandwidth >1 Tb/s combined with high bandwidth WDM links can have a very high cost. Furthermore, an all-optical switch can have high loss, so that it requires expensive optical amplifiers to compensate for the loss. There is therefore a need for an optical network architecture that supports many high-bandwidth inputs and outputs (>500) with non-blocking switching and minimal O-E-O conversion for the switching.
15. Network Management and Management Sublayers
Network functionality can be described by the 7-layer OSI model. Optical networking equipment resides mainly at the lowest layer, the Physical Layer. For the purposes of describing WDM networks in general and the current invention in particular, the Physical Layer can be divided into sublayers 1400 as shown in FIG. 14.
Except for the wavelength assignment and detection 1431, all of the sublayers shown are optional. For example, transponders do not necessarily provide electrical mapping, multiplexing, or protection switching.
The electrical sublayers 1420 include:                The mapping sublayer, where client data is received and mapped to available bandwidth according to the mapping protocol used.        The multiplexing sublayer, where electrical data is selectively added, dropped, or passed through.        The protection switching sublayer, which can provide protocol-based protection, e.g. UPSR or BLSR protection for SONET-mapped signals, or STP or RPR protection for ethernet-mapped signals.        The next 2 sublayers are typically implemented according to the ITU G.709 standard that defines OTN frame formats. Path trace and CRC checks can be inserted into the OTN frame for receive side monitoring of the signal source and signal quality respectively.        The lowest electrical sublayer provides forward-error correction encoding and correction.        
Of the optical sublayers 1430, the highest sublayer 1431, maps the signal from the client onto a specific wavelength that is routed over the network by fixed or tunable optical filters. The optical protection layer provides redundant optical paths from the source to destination and a means for bridging the traffic onto the redundant paths and selecting the received signal from one of the redundant paths according to alarms and signaling in the network. The lowest optical sublayer provides multiple point-to-point connection between two points according to the provisions in the higher layers.
Managing a WDM network requires that the network management system (NMS) have a management link 1440 from the NMS server to all of the optical network elements. The network connections can be provided by an external IP network, or with dedicated overhead channels that are provisioned on the optical network. The overhead channel may be mapped directly to one of the deployed wavelengths, or it may be transmitted over the OTN overhead channel, e.g. GCC0 in G.709, or in an unused section of the higher-layer protocol's overhead channel.
Software on the WDM equipment is required to configure, monitor, maintain, and report on all of the functions shown in FIG. 14. Adding new optical functionality requires adding new management software at the appropriate sublayer.
16. Ensuring Wavelength Accuracy in WDM Networks
In WDM networks, the laser wavelength (or frequency) must be maintained within a certain accuracy so that there is no interference between neighboring channels, and there are no penalties from laser-filter misalignment.
As is the case with all electronic and optical components, the performance characteristics of the lasers employed in DWDM systems change with temperature and with time. In particular, the frequency of emitted laser light changes due to ambient temperature variations (typically from −5 degC to 65 degC) and due to aging.
WDM laser frequencies are maintained to a first order by controlling the temperature of the laser by mounting the laser on a thermoelectric cooler (TEC). Etalons may also be integrated into the laser cavity to provide a second-order correction. Currently deployed WDM lasers have an accuracy that is adequate for 50 GHz spacing. There is currently a need for more accurate means of controlling laser frequencies to space the WDM channels as close together as possible.